1. Field of the Invention
The present invention relates to mobile communication equipment and, more particularly, to a surface-acoustic-wave duplexer for use in the radio-frequency section of, for example, a mobile telephone.
2. Description of the Related Art
Two types of surface-acoustic-wave (SAW) duplexers are known. In the first type, a transmitting SAW filter and a receiving SAW filter are formed on two separate piezoelectric substrates, which are mounted in separate cavities in a single duplexer package. In the second or monolithic type, a single piezoelectric substrate, on which both a transmitting SAW filter and a receiving SAW filter are formed, is mounted in the duplexer package. In the monolithic type of SAW duplexer, the shunt-arm SAW resonator of the transmitting SAW filter and the series-arm SAW resonator of the receiving SAW filter are conventionally disposed side by side.
FIG. 1 schematically illustrates the layout of a SAW duplexer of the monolithic type, wherein a transmitting SAW filter and a receiving SAW filter are formed on a single piezoelectric substrate or chip 10. These two filters include four SAW resonators, which are laid out in a row in the following order: a transmitting series-arm SAW resonator 11, a transmitting shunt-arm SAW resonator 12, a receiving series-arm SAW resonator 13, and a receiving shunt-arm SAW resonator 14. In this and subsequent drawings, Tx is used as an abbreviation for transmitting, Rx for receiving, S for series-arm, and P for shunt-arm.
When a monolithic SAW duplexer of this type is employed, problems of crosstalk from the transmitting circuits to the receiving circuits are observed. The transmit-to-receive isolation characteristic is influenced by many factors, such as the composite frequency characteristic of the signal path from the transmitting terminal to the antenna terminal and the signal path from the antenna terminal to the receiving terminal, the impedance characteristic of the signal path from the transmitting filter to the fifty-ohm (50-xcexa9) antenna termination to the receiving filter, and interference (coupling) due to the monolithic layout of the SAW resonators of the two filters.
In some mobile communication systems, such as the J-cdma (Japanese code division multiple access) system, the transmitting band and the receiving band are both divided in two. In this type of system, a duplexer with a full-band configuration, i.e., a duplexer that covers all parts of each band, requires comparatively wide passbands. For J-cdma, the passband width must be thirty-eight megahertz (38 MHz). This forces the transmitting band and the receiving band to be closely adjacent, and when the SAW resonators are all disposed on one chip, interference between them becomes inevitable.
The basic principle of a SAW filter is the propagation of surface waves on a piezoelectric substrate (e.g., LiTaO3) that is patterned to function as a bandpass filter by modal resonance. For a J-cdma mobile phone, the isolation characteristic of interest is that in the transmitting frequency band (887 MHz to 925 MHz). The receiving frequency characteristic in this band is dominated by the series-arm SAW resonator of the receiving filter. Since a two-chip duplexer is free of isolation problems and a monolithic duplexer is not, it is evident that the series-arm SAW resonator of the receiving filter is affected by interference from the adjacent shunt-arm SAW resonator of the transmitting filter.
By increasing the number of SAW resonators in the series arm of the receiving filter or the shunt arm of the transmitting filter, it is possible to increase the attenuation of one filter in the passband of the other filter. However, as long as the shunt-arm SAW resonators in the transmitting filter and the series-arm SAW resonators in the receiving filter are disposed side by side, some surface acoustic waves will propagate from the shunt-arm SAW resonators of the transmitting filter to the series-arm SAW resonators of the receiving filter on the surface of the piezoelectric substrate disposed between the shunt arm of the transmitting filter and the series arm of the receiving filter, degrading the isolation characteristic between the two filters by reducing the attenuation of each filter in the passband of the other filter.
A more detailed explanation of this problem will be given with reference to a simulated example. FIGS. 2, 3, 4, and 5 show characteristics, as determined by simulation, of SAW resonators of the general type employed in the present invention.
FIG. 2 shows the frequency characteristic of a series-arm SAW resonator. The attenuation loss is one-half decibel (xe2x88x920.5 dB) at a frequency of 860 MHz, xe2x88x922 dB at 870 MHz, xe2x88x924 dB at 880 MHz, xe2x88x9218 dB at 890 MHz, xe2x88x923 dB at 900 MHz, =2 dB at 910 MHz, and xe2x88x922 dB at 920 MHz. The return loss is xe2x88x9223 dB at 860 MHz, xe2x88x9211 dB at 870 MHz, xe2x88x923 dB at 880 MHz, xe2x88x922 dB at 890 MHz, xe2x88x923 dB at 900 MHz, xe2x88x926 dB at 910 MHz, and xe2x88x928 dB at 920 MHz.
FIG. 3 shows the reflection coefficient characteristic of a reflector that can be used in a series-arm SAW resonator. The reflection coefficient is 0.65 at a frequency of 860 MHz, 0.95 at 880 MHz, 0.95 at 900 MHz, and 0.95 at 920 MHz.
FIG. 4 shows the frequency characteristic of a shunt-arm SAW resonator. The attenuation loss is xe2x88x9213 dB at a frequency of 860 MHz, xe2x88x9224 dB at 870 MHz, xe2x88x924 dB at 880 MHz, xe2x88x922 dB at 890 MHz, xe2x88x922 dB at 900 MHz, xe2x88x922 dB at 910 MHz, and xe2x88x922 dB at 920 MHz. The return loss is xe2x88x922 dB at 860 MHz, xe2x88x922 dB at 870 MHz, xe2x88x925 dB at 880 MHz, xe2x88x9217 dB at 890 MHz, xe2x88x9210 dB at 900 MHz, xe2x88x927 dB at 910 MHz, and xe2x88x926 dB at 920 MHz.
FIG. 5 shows the reflection coefficient characteristic of a shunt-arm SAW resonator. The reflection coefficient is 0.55 at a frequency of 860 MHz, 0.95 at 880 MHz, 0.95 at 900 MHz, and 0.95 at 920 MHz. In the frequency band between 860 MHz and 840 MHz, however, the reflection coefficient is significantly reduced, becoming about 0.20 to 0.40, only about 30% of the reflection coefficient in the band between 860 MHz and 920 MHz.
The frequency characteristics and reflector reflection coefficient characteristics shown in FIGS. 2, 3, 4 and 5 were obtained by simulating the operation of, for example, a series-arm SAW resonator having one interdigital transducer (IDT) with an aperture of one hundred micrometers (100 xcexcm) and one hundred pairs of electrode fingers. The attenuation loss characteristics in FIGS. 2 and 4 are the characteristics of a resonator provided with a reflector.
FIG. 6 illustrates the frequency characteristic 15 of the transmitting filter and the frequency characteristic 16 the receiving filter in a conventional duplexer, indicating the resonance frequency fRxP of the shunt-arm SAW resonator in the receiving filter, the resonance frequency fRxS of the series-arm SAW resonator in the receiving filter, the resonance frequency fTxP of the shunt-arm SAW resonator in the transmitting filter, and the resonance frequency fTxS of the series-arm SAW resonator in the transmitting filter. FIG. 7 schematically illustrates the isolation characteristic of a SAW duplexer. In region A (shaded), which is the region in which interference occurs, the isolation characteristic is determined predominantly by the frequency characteristic of the series-arm SAW resonator transducer of the receiving filter.
In the series-arm SAW resonator shown in FIG. 2, the attenuation loss in the series resonance band centered at the lower resonance frequency of 860 MHz is only about 0.5 dB, indicating that little leakage of surface acoustic waves occurs at this frequency. As can be seen in FIG. 3, the reflection coefficient characteristic is about 60% or more, so the small amount of leakage is further reduced and causes little problem.
On the other hand, in the shunt-arm SAW resonator shown in FIG. 4, the attenuation loss in the series resonance band centered at the lower resonance frequency of 860 MHz is about 13 dB, and as shown in FIG. 5, the reflection coefficient characteristic at this frequency is about 30%, so considerably more leakage of surface acoustic waves occurs. This leakage from the shunt-arm SAW resonator appears as noise in the output of the series-arm SAW resonator.
As noted above, the reflection characteristic of the reflector in the frequency band below 860 MHz is about 30% of that in the frequency band above 860 MHz. Therefore, if the leakage of signals from the shunt-arm SAW resonator of the transmitting filter into the series-arm SAW resonator of the receiving filter increases, it becomes difficult for the series-arm SAW resonator of the receiving filter to generate a pole (a point where the attenuation increases sharply) in the band above 860 MHz (i.e., it becomes difficult to generate a pole to increase the attenuation). As a result, the frequency characteristic of the receiving filter in the band above 860 MHz is degraded (the attenuation is decreased).
The above problem of leakage or isolation can also be viewed as being due to the distance between the transducers in different resonators.
An object of the present invention is to provide a SAW duplexer in which a transmitting filter and a receiving filter are integrated on a single chip, while providing sufficient attenuation to meet isolation requirements in the transmitting frequency band.
The invented duplexer has a piezoelectric substrate on which a first filter and a second filter are integrated. The first filter includes a first SAW resonator and a second SAW resonator. The second filter includes a third SAW resonator and a fourth SAW resonator. The four SAW resonators comprise, for example, respective interdigital transducers with reflectors. Among the four SAW resonators, the first SAW resonator has the lowest resonance frequency and the fourth SAW resonator has the highest resonance frequency. At least one of the second and third SAW resonators is placed at one end of the piezoelectric substrate, adjacent to the first or fourth SAW resonator, so that the second and third SAW resonators are mutually separated. Preferably, the second and third SAW resonators are disposed at mutually opposite ends of the piezoelectric substrate.
In one configuration, the first filter is a ladder filter and the second SAW resonator is its series-arm resonator, while the second filter is a ladder filter and the third SAW resonator is its shunt-arm resonator. If the first filter is the receiving filter and the second filter is the transmitting filter, then either the shunt-arm SAW resonator of the receiving filter or the series-arm SAW resonator of the transmitting filter, or both of these SAW resonators, is disposed between the series-arm SAW resonator of the receiving filter and the shunt-arm SAW resonator of the transmitting filter. In a preferred configuration, the four SAW resonators are arranged in a row, with the series-arm SAW resonator of the lower-frequency filter followed by the shunt-arm SAW resonator of the lower-frequency filter, then the series-arm SAW resonator of the higher-frequency filter, then the shunt-arm SAW resonator of the higher-frequency filter.
Separation of the second and third SAW resonators provides sufficient isolation between the two filters, even if the resonance frequencies of the second and third SAW resonators are closely adjacent. The isolation may be further improved by providing a groove in the substrate between the first and second filters.